1. Field of the Invention
This invention relates to medical monitoring apparatus, and more particularly to a non-invasive apparatus for continuously monitoring and measuring various biophysical functions through tile finger of a patient.
2. Description of the Related Art
Non-invasive monitoring of biochemical and biophysical functions is largely focused on obtaining data from the fingers by coupling various biosensors capable of acquiring oxygen and carbon dioxide tension, oxygen saturation, PH and other measurements from the patient's skin. Biophysical measurements such as blood flow and blood pressure are also obtained. The measurement of blood pressure based on repetitive evaluation of the cutaneous blood pressure fluctuation patterns of minute branches of larger arteries reflects the arterial blood pressure of the general circulation since the latter is the source of the former.
The continuous monitoring of blood pressure patterns over extended periods of time is often needed in the evaluation of circulatory function, and is useful for hypertension studies and for obtaining records of circulation in the peripheral systems, particularly of the limbs, fingers and toes. Many devices currently used for patient monitoring are frequently attached for hours and/or days at a time.
To obtain accurate measurements of these biophysical and biochemical functions, the stability of the coupling between sensor and patient is of paramount importance. If instability is present, motion artifacts or "noise" may also contaminate the data that it is rendered inaccurate and unreliable.
At present, one of the most widely used measurements is skin oxygen saturation which is determined by analyzing the oxygen content of the blood flowing into a finger using optical methods such as infra-red radiation and receiving sensors. These sensors are held against the finger by "clothespin" type attachment devices where the retention element is a metal or plastic spring. Since finger size is not uniform, compensation is provided by lining the arms of the device with sponges or air-sacks. While the addition of these materials add to the stabilizing of the attachment system, they absorb mechanical energy so that the fidelity of the blood flow pulse shape is compromised. Furthermore, the skin-sensor interface is not completely stabilized since the anatomy of the finger is tapered as is the aperture provided by the "clothespin" attachment device.
The measurement of blood pressure and associated pressure pulse waveform demands a more stable coupling system. In the past, gantry type systems have been used with an isolation ring pressure transducer held firmly against the finger-tip with the aid of a longitudinally directed spring as disclosed in U.S. Pat. No. 5,025,792 issued Jun. 25, 1991 to Hon et al. Adjustment to individual patient's finger size was provided by a screw mechanism. Since the application system to a large extent was "customized" to the patient's anatomy the stability achieved was greater than that of a "clothespin" attachment device. However, since the adjustment to an individual patient's finger required the in-line insertion of a spring, some distortion of pressure pulse waveform was an inherent problem.
Stability of the attachment device is also of utmost importance when the sensor is an electrode. In medicine, electrodes are widely used for recording the electrocardiogram of a patient and may be grouped into two major types. The first group is the reusable electrocardiographic (ECG) electrodes which are attached to the patient's extremities with latex straps and to the patient's body with suction cups. The second group is the single use ECG electrodes which are attached to the patient's extremities with adhesive material. The conductive material of the reusable electrodes is usually metallic such as silver or nickel silver and is cleansed between uses with an antiseptic agent such as alcohol. The conductive material of the single use ECG electrodes is usually a silver-silver chloride composite or silver-silver chloride alone or some solid state ionic conducting material. A conductive gel or paste covers the surface of the electrode which is isolated from the patient by a sponge.
However, both types of ECG electrodes have disadvantages. The reusable type are inconvenient to use and can only be used for short intervals. With exercise the suction cups become loose and electrical contact is lost. Furthermore, the metallic nature of the electrode material is not as compatible with human tissue as a metallic salt electrode such as silver-silver chloride. If silver is used as the electrode material, chloride ions from the sodium chloride present in body perspiration gradually will deposit on the silver reducing the effective surface area and thus increasing electrode resistance. Additionally the direct metal-to-skin electrode contact generates electrical "noise" with skin movement against the electrode resulting in motion artifacts in the record rendering the data obtained inaccurate and unreliable.
These disadvantages are largely eliminated by the single use electrode where a silver-silver chloride electrode is indirectly coupled to the patient by a gel-salt bridge and the electrode is attached to the patient's body with an adhesive backed sponge disk. However, this type of electrode often encounters a problem with the adhesion during initial attachment if the patients skin is damp. This problem is increased in some patients during various exercise tests used for monitoring cardiac function. This is especially true in "sports" medicine where athletes are exercised to maximum limits. One other problem with the adhesive type electrode is an allergic reaction to the adhesive material.
Therefore, there exists a need for a highly stable attachment device used to couple sensing monitors to a patient for a prolonged period without encountering the noise and motion artifacts experienced by the attachment devices of the past.